The invention relates to a pneumatic brake booster, particularly for a motor vehicle.
Such a booster comprises a rigid casing in which two chambers are delimited and separated from one another by a moving transverse partition which bears an axial piston on which there acts a plunger mounted at the end of a control rod actuated by a brake pedal and which itself acts via a push rod on a piston of a brake master cylinder. A front chamber of the booster is connected to a source of vacuum and the rear chamber is connected respectively either to the front chamber or to the surrounding atmosphere.
The rear end of the plunger collaborates with an annular shutter of a three-way valve, which is mounted at the rear end of the booster around the control rod and allows the two chambers of the booster to be made to communicate in the rest position, that is to say when there is no braking, and allows them to be isolated from one another then allows the rear chamber to be connected to the surrounding atmosphere under braking.
The atmospheric pressure let into the rear chamber acts on the piston and moves it forward, amplifying the effort applied by the driver to the brake pedal and transmitted by the control rod.
Under normal braking, the control rod and the piston of the booster move at the same rate. When the control rod is in the forward position, the plunger that it bears is pressed against a reaction disk made of a compressible material, such as rubber or an elastomer, mounted in a dish at the rear end of the push rod. The resistive force of the brake circuit is thus passed back by the plunger and the control rod to the brake pedal, and this helps the driver to modulate the effort he is applying to the brake pedal to suit the desired conditions of braking for the vehicle.
Under emergency braking, the control rod and the plunger are made to move forward more quickly than the piston of the booster, and the feedback of force transmitted to the brake pedal may be great even though the booster has not yet provided maximum boost. This may lead the driver to relax the effort he is exerting on the brake pedal prematurely even if, at that moment, the braking effort needs to remain at a maximum.
It has already been proposed for these boosters to be equipped with emergency brake assist means which, when the control rod has been moved to an extreme forward position or into close proximity of this extreme forward position, reduces quite sharply the reaction of the braking circuit on the control rod when the driver partially relaxes the effort applied to the pedal. If the driver has then to press the pedal firmly again, something which often occurs in practice, he can more easily return the control rod to the extreme forward position for maximum braking effort.
These emergency brake assist means generally comprise a feeler mounted to slide at the front end of the plunger and associated with means of axial immobilization with respect to the piston of the booster, making it possible, when the control rod has been brought into an extreme forward position and the driver has partially relaxed the effort exerted on the brake pedal, to cause the piston to withstand at least some of the reaction of the braking circuit without transmitting it to the plunger and to the control rod.
It is a particular object of the invention to improve these emergency brake assist means so that the assistance provided by the booster in the aforementioned case where the driver partially relaxes the effort applied to the brake pedal then reapplies significant effort to this pedal, increases and is a function of the rate of travel of the control rod in the preceding phase of the braking action during which the control rod was moved into its extreme forward position or into close proximity thereof.
To this end, the invention proposes a pneumatic brake booster particularly for a motor vehicle comprising an axial piston interposed between a push rod and a plunger mounted at the end of a control rod, and emergency brake assist means comprising a feeler borne by the plunger and able to be moved in axial sliding with respect to the latter and means of axially immobilizing the feeler characterized in that these immobilizing means are able to block the feeler with respect to the plunger at rest and under braking when the rate of travel of the control rod and of the plunger is below a limit value, and to allow the plunger to slide along the feeler in the direction of reducing the axial length of the plunger and feeler as a whole under emergency braking when the rate of travel of the control rod and of the feeler is above the said limit value.
By virtue of the feeler being blocked along the plunger at rest and under normal braking and of the unblocking that allows the plunger to slide along the feeler under emergency braking, the reduction in overall length of the plunger and feeler as a whole depends on the rate of travel of the control rod and of the plunger under emergency braking. This reduction in the overall length of the plunger and feeler as a whole results in an increase in the jump phase well known to those skilled in the art, and therefore in an increase in the amount of boost supplied by the booster.
According to other features of the invention:                the feeler comprises means collaborating with the piston to define a position of rest of the plunger,        these means comprising a pin mounted in a transverse orifice of the feeler,        the ends of the pin are housed in oblong slots of a cylindrical part of the piston, these slots delimiting a maximum axial travel of the feeler with respect to the piston,        the pin passes through at least one oblong slot formed in the plunger and defining a rest position of the plunger with respect to the booster piston.        
In a first embodiment of the invention, the feeler is housed and guided in axial sliding in a cylindrical axial passage in the opposite end of the plunger to the push rod, and the plunger itself is guided in an axial cylindrical passage of the piston comprising, on the same side as the push rod, a radial wall formed with an axial orifice for the passage of the feeler.
In this embodiment, the feeler immobilizing means comprise a pivoting key mounted in a transverse housing of the piston and comprising an orifice through which the feeler passes with clearance, and elastic return means constantly urging the key to bear against the plunger.
This key can be moved by the plunger, under emergency braking, between a position of blocking the feeler and a position of bearing on the piston of the booster, in which position the key relaxes the feeler and allows the plunger to slide along the feeler in the direction of the push rod, in order to reduce the overall length of the plunger-feeler assembly.
In another embodiment of the invention, the plunger is housed and guided in sliding in an axial passage of the feeler, itself housed and guided in sliding in an axial passage of the booster piston.
In this embodiment, the immobilizing means comprise ramp or wedge means mounted between a cylindrical surface of the plunger and an internal frustoconical surface of the feeler.
Advantageously, these immobilizing means are formed of a plurality of tapered rollers that are distributed in a ring around the cylindrical surface of the plunger and that are held axially by two rings, or washers slipped over the plunger.
These immobilizing means are urged axially toward the push rod by elastic return means bearing against a shoulder of the plunger, and are pressed onto a pin engaged in transverse slots of the plunger, of the feeler and of the piston.
The pin defines a position of rest of the plunger and of the feeler when it is pressed against one end of the slots of the piston at the same end as the control rod.
The feeler itself is urged axially by elastic return means in a direction away from the push rod.
The invention will be better understood and other features, details and advantages thereof will become more clearly apparent in the light of the description which follows, given by way of example with reference to the attached drawings in which: